The invention relates to a picture display device comprising a display tube having a display screen and an electron gun facing said screen and having a cathode centred along an electronoptical axis and a plurality of electrodes which jointly constitute a beam-shaping part for generating an electron beam, said gun further comprising a tubular structure having an outer surface and an inner surface on which a material having a high electrical resistance is provided, from which a helical resistance structure is formed which constitutes a focusing lens, said tubular structure having a coaxial input portion and a coaxial output portion.
A focusing lens formed from a high-ohmic resistance layer and having a helical structure for obtaining a low spherical aberration for use in display tubes is known.
Correction elements may also be formed in the high-ohmic resistance layer in front of, between or behind the focusing lens, which elements generate electric multipoles (such as 2-poles, 4-poles, 6-poles, 8-poles). However, it appears that problems occur if a dynamic correction signal is to be applied to these correction elements. The very high resistance of the resistance layer (the resistance per square of specific layers may range between 10.sup.6 and 10.sup.8 ohm) causes problems when applying dynamic correction signals, particularly if the frequency of the correction signal exceeds 16 kHz. This is a result of the large intrinsic RC time of the layer.
Since the material of the resistance layer has such a high electrical resistance (for example, 10 GQ) the RC time is large (for example, 10 msec). As a result, the effect of a dynamic correction voltage hardly penetrates the resistance structure. One of the objects of the invention is to provide a picture display tube comprising an electron gun with a focusing lens of the above-mentioned type which is suitable for using dynamic corrections.